[This isn't done yet, but I've shown it to enough people that I should share it with the list.]

1. Introduction and background

1.1. Motivation

Frequently, we find that very old versions of Tor should no longer be supported on the network. To remove relays is easy enough: we simply update the directory authorities to stop listing them.

But to disable clients is harder. We can do as we did with v1 directories in Tor 0.2.0.5-alpha, and make them know about no relays at all.

Stepping back -- why should we disable very old clients at all? Here are some reasons why:

* Security. Anybody who hasn't updated their Tor client in 5 years is probably vulnerable to who-knows-what attacks. They aren't likely to get anonymity either.

* Withstand zombie installations. Some Tors out there were once configured to start-on-boot on now-unmaintained systems. They put needless load on the network, and can't be easily disposed of.

* Disable backward-compatibility code. Currently, Tor supports some truly ancient protocols in order to avoid breaking ancient versions. If we disable these protocols without turning off the clients that use them, we run the risk that those clients will run wild, retrying their connections to the network over and over.

This document describes a designs for use with future versions of Tor to better support client deprecation and removal.

It also describes how we can safely disable existing client versions that do not support this proposal.

1.2. History

Early versions of Tor checked the recommended-versions field in the directory to see whether they should keep running. If they didn't recognize

1.3. Goals and requirements

We need older clients to stop putting (much) load on the network. Having them stop working entirely is not sufficient: they need to stop working in such a way that they don't keep using up resources.

In the future, when we disable a client, it would be a good thing to provide some means to warn users in advance that they need to upgrade, and to provide advice about doing so.

It might be good to make any disable-switches advisory rather than mandatory.

2. Adding a disable-clients switch to consensus documents

I specify a new consensus element: disable-versions. It may appear any number of times in a consensus document. The syntax is:

disable-versions SP SOFTWARE SP ROLE SP DATE SP VERSIONS

DATE = A date in YYYY-MM-DD format ROLE = "relay", or "all".

SOFTWARE = "Tor"

VERSIONS = VERSION_RANGE VERSIONS = VERSIONS SP VERSION_RANGE

VERSION_RANGE = VERSION VERSION_RANGE = VERSION..VERSION

[TODO: Specify voting.]

If the current version is listed in any disabled-versions line for its current role, then we should warn the user. If the current date is on after the date specified (starting midnight UTC), we should disable the host as follows:

* If 'relay' is the disabled role, then we should enter ClientOnly mode.

* If 'all' is the disabled role, we should enter DisableNetwork mode.

Note that the DisableNetwork behavior means that once a client is disabled, it will not fetch new consensuses.

To override this, a Tor instance may include a configuration option, "IgnoreDisabledVersion VERSION". It is an error to set this option when *not* running with a disabled version.

[ALTERNATIVE:

Make RequireLinkProtocol, RequireCircuitProtocol, etc lines for the consensus. Any Tor not supporting those protocols should behave as above. This allows non-Tor implementations to respect these fields.]

3. Disabling current client versions.

3.1. Disabling versions that use the v2 handshake

These versions (before 0.2.3.6-alpha) use a renegotiation-based handshake instead of our current handshake.

When we detect a renegotiation attempt, we can close the connection, or stop reading on it.

3.2. Disabling versions prior to 0.2.4.11-alpha.

Starting in version Tor 0.2.4.11-alpha, we implemented proposal 214. This proposal added a new link protocol, version 4. Earlier version of Tor never support it; newer versions of Tor always advertise their support for it.

We could treat the presence of any link protocol of version 4 or later as indicating the presence of 0.2.4.11-alpha. Upon receiving a connection from such a version, we could throttle the bytes we give it, drop its circuits, or stop reading.

[TODO: which?]

3.3. Disabling versions prior to 0.2.5.x

In tor 0.2.5.1-alpha, we began ignoring the UseNTorHandshake option, and always preferring the ntor handshake where available.

Unfortunately, we can't simply drop all TAP handshakes, since clients and relays can still use them in the hidden service protocol. But we could detect these versions by:

Looking for use of a TAP handshake from an IP not associated with with any known relay, or on a connection where the client did not authenticate. (This could be from a bridge, but clients don't build circuits that go to an IntroPoint or RendPoint directly after a bridge.)

In response, we can throttle the connection, drop all its circuits, or stop reading on it. [TODO: which?]

3.4. Disabling all versions that don't support this proposal

We should allow 'n' (short for "node") as a synonym for 'r' in consensus documents. Later, if we want to disable all Tor versions before today, we can change the consensus algorithm so that the consensus (or perhaps only the microdesc consensus) is spelled with 'n' lines instead of 'r' lines. This will create a consensus which older clients and relays parse as having no nodes.

3.5. In the event of overzealous retries

We need to be sure that client running versions from 0.2.1 through 0.2.6 respond gracefully to the responses above. In particular, we need to make sure that they don't continually retry the connections that fail in these ways: that would put a lot of extra load on the network.

Above, I recommend stalling connections rather than just closing them. This may prevent the risk of retries, at the risk of using additional relay resources.

To handle something like this, it could be necessary to use the trick of 3.4 above.